At the present time, automotive exhaust systems equipped with catalytic conversion systems generally comprise an exhaust line connecting with a converter housing, the housing enclosing a ceramic or metal honeycomb substrate supporting an oxidation or, more commonly, a three-way emissions control catalyst. The three-way catalyst operates to oxidize carbon monoxide and unburned hydrocarbons present in the exhaust stream and, with proper control of exhaust stream stoichiometry, to at least partially reduce higher oxides of nitrogen (NOx) contained therein.
Tightened emissions standards for automotive gasoline engines have placed higher demands on the performance of these catalytic conversion systems. Particularly critical for overall system performance is performance during the so-called "cold start" phase of engine operation. This is the period of engine operation covering approximately the first 60 seconds after cold engine start and prior to startup or "light off" of the catalytic converter, during which the highest concentrations of unburned hydrocarbons are released into the atmosphere.
One type of exhaust system designed specifically to address the cold-start problem provides a hydrocarbon adsorber in the exhaust line. The adsorber operates to trap unburned hydrocarbons emitted during engine startup, and then to release those hydrocarbons to a catalytic converter after converter light-off has been achieved. A preferred configuration for an adsorber in such systems is a honeycomb structure, similar in construction to a catalyst support honeycomb but composed of, or supporting, a coating of a hydrocarbon adsorbent such as carbon, zeolite, or another molecular sieve material.
Examples of recently developed cold-start engine emissions control systems of this type are disclosed in published patent applications WO 95/18292, EP 0661098 and EP 0697505 (Hertl et al). Two further examples of such system are described in co-pending, commonly assigned U.S patent application Ser. No. 08/578,003 (Brown et al.) filed Dec. 22, 1995 and entitled "Exhaust System with a Negative Flow Fluidics Apparatus" and U.S. patent application Ser. No. 08/685,130 of J. Anderson et al. filed Jul. 24, 1996 and entitled "Exhaust Gas Fluidics Apparatus".
A common feature of these systems is a ported honeycomb adsorber, i.e., an adsorber comprising a by-pass port integral with its structure, located downstream of a main or light-off catalytic converter but positioned upstream of a second or so-called "burn-off" catalytic converter. This adsorber functions to trap the hydrocarbons released at engine startup and slowly desorb and release the hydrocarbons to the burn-off converter as the adsorber is heated by the warming exhaust gases. A particular advantage of the ported adsorber design is the faster light-off of the burn-off converter due to exposure of that converter to the hot exhaust gases passing directly through the adsorber port. In the design of Hertl et al., Brown et. al. and Anderson et al., control the flow of the exhaust gases through or past the adsorber is secured by means of a fluidic diverter which delivers a control gas stream for diverting the exhaust gases toward or away from the adsorber port in the course of engine operation.
Although these cold-start adsorber exhaust systems provide improved emissions performance, (i.e., during the adsorption phase) the resulting flow characteristics for this system when the fluidics diverter is operational are not completely ideal and are somewhat unstable, as they are quite dependent on engine conditions and diverter/honeycomb positioning. Specifically, under adsorbing/diversion conditions, the flow pattern across the inlet face of the actual adsorber tends to somewhat non-uniform; i.e., a high flow volume in the adsorber cells proximate to the centrally located by-pass port and a reduced flow volume in those cells nearer the edge of the adsorber. Furthermore, the instability of the flow in these systems may result in exhaust flows wherein a portion of the exhaust gas continues to pass through centrally-located by-pass port, thus resulting in hydrocarbons which pass through un-adsorbed. This being the case, work has continued to discover exhaust systems exhibiting more stable, improved exhaust flow characteristics and performance.